<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f1()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f3()">
<input type="button" value="除" onclick="f4()">
<h3>
    结果:<span></span>
</h3>

<script>
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let s = document.querySelector("span");

    function f1() {
        f();
        s.innerText = parseFloat(i1.value) + parseFloat(i2.value);
    }

    function f2() {
        f();
        s.innerText = i1.value - i2.value;
    }

    function f3() {
        f();
        s.innerText = i1.value * i2.value;
    }

    function f4() {
        f();
        s.innerText = i1.value / i2.value;
    }

    function f() {
        if (isNaN(i1.value) || isNaN(i2.value)) {
            s.innerText = "输入错误!";
            return;
        }
    }

</script>
</body>
</html>